As the evolution of computers has progressed, so has the evolution of hard disk drives. The vast majority of hard disk drives are now manufactured with "flying" heads attached to an actuator arm. The "flying" head consists of a slider supporting write and read heads to write data to and read data from an adjacent disk over which the slider flies. Also associated with "flying" head are parking zones. Parking zones allow the "flying" head to be safely landed after the hard drive has ceased operation. During the power down stage of a hard disk drive, the flying head is moved to a parking zone before the hard disk drive stops spinning. Otherwise the head will land outside the parking zone, potentially damaging the disk.
The parking zone on a hard disk drive varies depending on the type of disk drive present. In some disk drives the designated parking zone is located on the disk, usually at the innermost track on the disk. More recently, parking zones have included ramps to raise the flying head and park it off the disk in an elevated position. The ramp is usually located at the outermost edge of the disk. During the power down stage, the actuator arm is moved to the outermost edge of the disk and up the ramp to the parking zone. The ramp parking zone offers certain advantages over disk parking zones because it does not waste valuable disk space, avoids problems of stiction between the head and disk, and reduces wear on the head due to disk contact, thus improves reliability of the disk drive.
When a disk drive is powered down, it usually performs certain operations before actually disconnecting from the external power source. One of these power down operations is to operate the actuator arm to move the head to the parking zone. However, in the event of a catastrophic shut down (i.e. external power is suddenly removed) there is no external power to perform power down procedures, including to move the head to the parking zone. Typically, the momentum of the spinning disk operates the spindle motor to generate a back electromotive force (BEMF) at the motor terminals, which is rectified and stored in a capacitor to provide power to the actuator control circuitry upon a catastrophic shut down. However, the voltage available to power the actuator is limited by the BEMF of the motor.
Bahlmann et al. (in U.S. Pat. No. 5,495,372) increases the voltage by short circuiting the motor inductance to charge the capacitor. As a result, Bahlmann et al. increases the voltage available to provide a low power to the actuator, which is sufficient for some disk drive systems. However, the Bahlmann et al. circuit does not provide sufficient power to operate the actuator up the ramp. Menegoli (in U.S. Pat. No. 5,504,402) improves upon Bahlmann et al. by monitoring and controlling the motor current to generate a higher current for operation of the actuator motor. Menegoli extracts power from the spinning motor by opening all commutator switches and short circuiting the stator coils through an additional switch in parallel with the commutator switch pairs. BEMF voltage is pulled through the inherent diode associated with the open upper switch associated with the terminal having the highest voltage. Alternatively, Menegoli operates the commutator's upper switches to eliminate the diode drop associated with upper switch inherent diodes. In either case, the Menegoli approach requires additional circuitry which must be powered by the BEMF during the catastrophic condition. Menegoli additionally requires that the storage capacitor be charged through at least one series diode to isolate the shorting switch from the load; the diode significantly reduces the power conversion efficiency.